The present invention relates to a method and an arrangement for stopping sectional drive controllably in connection with a fault situation in the supplying mains, the sectional drive comprising a plurality of inverters which are supplied by a DC voltage intermediate circuit supplied by the mains, a plurality of electric motors which are controlled by the inverters with speed and/or torque control and which are used for implementing speed and tightness reference values of the strip in the sectional drive, the tightness reference value being predetermined motor-specifically or motor-group-specifically.
Entities comprising a plurality of motors, where motors are used for moving a material strip, are referred to as sectional drive. In sectional drives, a plurality of motors is synchronously controlled in such a way that the strip remains as desired. The properties of the strip in connection with sectional drive include for instance the speed and tightness of the strip. The most typical examples of the sectional drive are, for instance, a paper machine and a metal-processing plant, where the paper web and the metal strip are maintained controllably, driven by a large number of motors.
The most typical way to implement controllable sectional drive is the use of alternating-current motors controlled by an inverter. Inverters allow control of the motor speed and the torque in a reliable manner, whereby the speed and tightness of the material strip can be maintained as desired.
In connection with a metal-processing plant, the problem in the sectional drive is that the process section stops uncontrollably as a result of a fault in the supplying mains, such as a power failure. In a metal-processing plant, a thin metal sheet is processed, which is rolled as a continuous strip through the process section. As part of the process, metal is heated in a furnace, the length of which is for instance between 50 and 100 meters and the temperature of which is between 700° C. and 1,000° C. Hereby, the composition of the metal strip, the thickness of which may be, for instance, between 0.15 and 10 mm, becomes plastic and thus also easily breakable.
In connection with present-day apparatus, when a sudden and surprising mains break occurs, for instance as a result of lightning, the motors stop uncontrollably, whereby the torques and speeds provided by the motors are not controllable. This leads to a situation where the metal strip breaks into the inside of the furnace. Since a furnace is a large construction, its cooling for cleaning and reheating to the process temperature for the purpose of starting the process takes a considerably long time. The length of a production down time caused by such a process failure may well be 24 hours, whereby the decrease in the production resulting from the down time is economically considerable.
Publication U.S. Pat. No. 5,196,769 discloses deceleration of an individual motor in such a way that its rotating mass is decelerated during a mains break to maintain the synchronism between the motor to be decelerated and the other motors. Said publication thus contains one motor with large inertia, which motor is controlled with speed control, the intention being to limit the speed of the motor in connection with a mains break.
Publication EP 836 764 also relates to limiting the speed during a mains break. In the solution according to the publication, apparatus with greatest mechanical energies are selected beforehand, the other motors being supplied with the energy of these particular apparatus to stop the motors synchronically.